Speculation

Some knights appear to be advanced (elite) versions of other troops. Stanley the Tool's Knights were drawn from all the (Hobgobwin) regiments in his (former) empire. Transylvito's Skanks are just a special class of Guard Goyles. It is possible that units of lower classes might be promoted to Knight. Archons do not seem to have a non-elite version and are known to be popped directly as knight-class units.

Perhaps Knights are always able to ride Mounts (thus the name Knight). If so, they must always be advanced forms of non-Heavy Units, and must not become Heavy Units when they become Knights. If so, then there would be no such thing as a Twoll Knight, for instance. Normally Infantry, by definition, fights on foot, yet we saw some of Stanley's Hobgobwin Knights ride on Dwagons. So it is possible that Infantry are unable to use Mounts, while Knights and Warlords may do so.

One recent exception to Knights riding mounts is in the case of the Hobgobwin Heavy Knights, such as Fud. They've been confirmed to retain their class upon promotion to Heavy Unit. However, Dwagons are not capable of flight if their rider is a Heavy Unit. If a knight is promoted to a heavy knight while riding a Dwagon that is in flight, the dwagon will fall to the ground, unable to support the new weight difference.

In the battle between Queen Jillian and Tool Stanley, Parson did not consider the Faq gwiffons to be much of a threat due to their riders not being Knights or lancers. We may therefore infer that Knights are much more effective than regular troops in mounted combat, though it is unclear whether this is a reflection of generally higher stats or special mounted combat bonuses.